“What goes on in San Diego is up to the people of San Diego,” Mrs. Pelosi said. “I’m not here to make any judgments — or even be fully versed on what happened.”

Rep. Charles B. Rangel, the veteran New York Democrat who was censured by the full House for a series of tax and ethics violations in 2010, brushed aside Mr. Weiner’s latest sexual scandal and its impact on the New York mayoral race.

“Knowing New York as I do — and I do know New York — this is not going to be a story by the time we get to September the 10th,” Mr. Rangel told MSNBC in an interview Wednesday.

Mrs. Pelosi also has refused to demand that Mr. Weiner exit the mayor’s race, saying that the decision should be left to the voters, according to a report by The Associated Press.

In 2006, however, Mrs. Pelosi called for a criminal investigation into former Rep. Mark Foley, Florida Republican, after he admitted to sending sexually explicit messages to an underage male congressional page.

In a statement, she said the investigation was warranted because members of Congress operate under a “special trust” with the pages. Even so, critics have accused Mrs. Pelosi of employing a double standard for Democratic sexual misbehavior.

Mixed record

While gleefully pointing up Republican sexual and influence-peddling failings before Mr. Obama came to power, Democrats have proved to be vulnerable to the same temptations under Mr. Obama. Good-government groups have tracked the rising number of former Obama aides who have found lucrative lobbying posts, and the number of top Democratic fundraisers rewarded with plum diplomatic posts.

Mr. Rangel’s ethical problems effectively cost him the chairmanship of the House Ways and Means Committee in 2010, just months before Democrats lost their majority in the chamber. Jesse Jackson Jr., an Illinois Democrat once considered — like Mr. Weiner — a rising force in his party’s House caucus — pleaded guilty in February to wire and mail fraud after he resigned his seat in November.

Democratic one-party rule in city halls across the country also has led to overreach and scandal. Detroit’s recent bankruptcy filing has trained another spotlight the corruption that plagued the administration of former Mayor Kwame Kilpatrick, a Democrat who went to jail after being convicted of charges ranging from mail fraud to racketeering over his six-year term that ended in 2008.

In the District of Columbia, former D.C. Council member Michael A. Brown last month pleaded guilty to a public corruption charge for accepting tens of thousands of dollars in cash stuffed in duffel bags and coffee cups while in office. He pleaded guilty after the chairman of the D.C. Council, Kwame R. Brown, resigned and pleaded guilty to bank fraud in June 2012. Former council member Harry Thomas Jr. resigned from the D.C. Council and pleaded guilty in January 2012 to stealing $350,000 in public funds intended for youth sports programs.

Critics say the Democrats’ sharp rhetoric when out of power has made it more difficult for figures such as Mrs. Pelosi to distance themselves from party members like Mr. Filner when they find themselves in trouble.

“Whether she’s carrying a gavel or just carrying water, Pelosi has served as a loyal apologist and abettor of the Democratic Bad Boys Club,” conservative columnist Michelle Malkin said in a Wednesday article. “What exactly will it take before voters finally turn this perv protector into a ‘former colleague’?”

Although the sexual scandals may be embarrassing for Democrats, Republicans have endured cringe-worthy moments from their own former lawmakers, notably former South Carolina Gov. Mark Sanford, who was elected to Congress in May despite revelations about an extramarital affair.

“You’re always going to have unscrupulous individuals elected to Congress who do unethical things. Republicans had Mark Sanford — Democrats have Anthony Weiner,” said Republican political consultant Dick Wadhams.